1. Field of the Invention
The present invention relates to a chip tester used for electrically testing electronic chip parts such as semiconductor bare chips before packaging them into devices. Each of the chip parts is disposed on a stage of the chip tester and brought into contact with a probe head of the chip tester such that pad-electrodes of the chip are made in contact with a plurality of pin-electrodes of the probe head and test signals are supplied through the probe head to the chip, thereby conventional electrical tests and dynamic performance tests being performed. Bare chips without packaging are more popularly used for multi-chip modules (MCM) in recent years, and this trend necessitates a chip tester with features of high handling efficiency and measurement precision.
2. Description of the Related Art
An exemplary chip tester of the prior art is shown in FIG. 1 which shows a schematic side view of the chip tester. A frame 10 is arranged on a main table 5 of the chip tester. The frame 10 comprises a plate 10a which is movable vertically with aids of support guides 10b and a vertical drive mechanism 4 comprising a shaft 12a and an air cylinder 12. On a bottom surface of the plate 10a, an X-Y stage 6 is arranged on an X-table 7 and a Y-table 8 which are movable in a horizontal plane, i.e., in an X-direction and in a Y-direction respectively, and can be rotated in a .theta. direction. A suction chuck (not shown) is provided on a surface of the X-Y stage 6, which holds a semiconductor chip 1 for test, thereby a chip surface with pad-electrodes looking downward.
An ITV camera 15 looking upward is arranged on a left side of the main table 5 and an image of the chip surface can be viewed on a screen of a monitor (not shown). Chip position on the X-Y stage 6 is adjusted to a specified position by respective drive mechanisms attached to the X-table 7 and Y-table 8. Thereafter, the frame 10 with the semiconductor chip 1 is subjected to a translational movement to a right side of the main table 5 (to a position shown by dashed lines) by a drive mechanism comprising a drive motor unit 40, a drive shaft 41 and a bearing unit 42. The chip surface on the X-Y stage 6 is thereby moved so as to position just above a probe head 20 which is arranged on the main table 5. The plate 10a is lowered by the vertical drive mechanism 4 so that pin-electrodes of the probe head 20 contact with the pad-electrodes of the chip 1. Test signals from a signal source 17 are supplied to the probe head 20 and the chip 1 is tested for a variety of tests.
The tester of the prior art as described above requires precise position alignment of the pad-electrodes of the chip with the pin-electrodes of the probe head 20. Therefore, correct position adjustment of the X-Y stage 6 using the X-table 7 and the Y-table 8 and correct control of the translational movement of the frame 10 are required such that each chip 1 is moved just above the probe head 20.
Another idea of alignment between a chip and a substrate is disclosed in Japanese laid-open Patent Publication Hei-2-244649, which idea is schematically shown in FIG. 2. FIG. 2 shows a schematic view comprising an optical head 44, a monitor 16 and a control system (not shown). The optical head 44 is inserted between the chip 1 and the probe head 20. The optical head 44 can pick-up and transmit both images of the pad-electrodes and the pin-electrodes to a monitor 16 through a fiber cable, and the images can be displayed on a screen 16a of the monitor.
FIG. 3 shows images on the screen 16a. On a left side of FIG. 3, the images of the pad-electrodes 101 of the chip 1 and the pin-electrodes 21 of the prove head 20 are separately illustrated. On the actual screen 16a of the monitor 16, a composite image of these two images is displayed. The chip position on the X-Y stage 6 is adjusted by the drive mechanisms of such as of X-table and Y-table in the X- and Y-directions such that positions of the pad electrodes 101 of chip 1 are coincident with those of the pin-electrodes 21 of probe head 20 as shown on a right side of FIG. 3. Thereafter, the optical head 44 is removed aside, and the X-Y stage 6 is lowered so that the pin-electrodes 21 are in contact with the pad-electrodes 101.
The chip tester of the prior art shown in FIG. 1 includes a problem that the translational movement of the frame 10 in which the chip 1 is arranged is liable to cause a positional error and each chip needs a fine position adjustment, which reduce a work efficiency of the test steps. Further, if anyone of pin-electrodes 21 includes a position shift, a tip of the pin-electrode strikes on an edge of the pad-electrode 101 giving a defect to the pad-electrode.
The chip tester of FIG. 2 includes the optical head 44 and an associated control system which are expensive, and the handling thereof is complicated.
Another factor related with the chip tester is a structure of the prove head. FIG. 4 shows an exemplary structure of the prior probe head 20 partly in cross section. The probe head 20 comprises a circular disk 201 of an insulation material having a center hole 202. A plurality of pads 203 are formed at regular intervals on a peripheral portion on a bottom surface of the circular disk 201. A pin guide 204 composed of parts 204a, 204b, 204c is arranged on the disk 201, each pin-electrode 21, one end of which is connected to each one of pads 203, is so arranged to penetrate through the center hole 202 and through each respective hole 205 of the pin guide part 204a. A pin electrode 21 is firmly fixed to the part 204c so that a projection length from a surface of the part 204c is a specified height H. The height H is so determined as to allow some bend of a vertical portion of the pin-electrode 21 and to obtain a pressed-contact when contacts between the pad-electrodes 101 and tips of the pin-electrodes 21 are formed.
When the chip tester and the probe head 20 as described above are used for tests, the pad-electrode 101 is sometimes damaged when the alignment is not accurate so that the tip of the pin-electrode strikes an edge of the pad-electrode 101. Further in a dynamic test, especially in a high frequency test, the test can not avoid an effect of reflection caused by an impedance mismatch at the probe head 20, resulting In deteriorating precision of the test.